The present invention relates to mirrors, and more particularly relates to an electrochromic mirror for passenger vehicles having an electrochromic mirror subassembly with thin glass elements that save weight, but are potentially so thin that they require support to prevent undesired breakage, provide adequate impact strength, and to prevent unacceptable flexure resulting in image distortion.
Electrochromic mirrors have gained wide acceptance in modern passenger vehicles due to their ability to darken as a way of reducing glare from other vehicle headlights. Typically, electrochromic mirrors include a pair of glass elements with electrochromic material therebetween that can be darkened to reduce the brightness of light reflected from a reflector material associated with the rear glass element. Usually, the glass elements are each at least about 2.2 mm thick so that the glass elements have sufficient internal structure to be self-supporting and resistant to flexure that would unacceptably distort the glass elements and result in distorted reflected images. Glass elements that are thinner than 2.2 mm traditionally are so thin that they may unacceptably deform, crack, or break. This can occur as a result of many different factors, such as from a person cleaning or pressing on the front glass element, or from impact during a vehicle crash, or from stresses generated within the mirror itself. Such stress can come from assembly, from non-uniform thermal expansion of components in the mirror, or from non-uniform support of the glass elements.
Despite the problems associated with thin glass elements, thin glass is very desirable because it results in significant mass reduction in mirror assemblies, which in turn, results in reduced mirror vibration and hence improved mirror function. Specifically, reduced mirror mass contributes directly to a reduction in a condition sometimes referred to as a "pendulum effect" which results in mirror vibration. The "pendulum effect" is caused by a mirror assembly having a mass supported in cantilever off of a vehicle front windshield. The vibration of the mirror increases as the mass of the mirror increases and as a length of the cantilever arm increases. The glass elements are good candidates for mass reduction because two glass elements are used and further, both are located at a front of the mirror assembly, at a point farthest from the vehicle front windshield where the cantilever arm is the greatest. In regard to vehicle weight, it is noted that vehicle manufacturers are extremely interested in reducing vehicle weight, even in small amounts. A reason for this is because reduced vehicle weight has several benefits, including improved vehicle gas mileage, improved/reduced emissions, more favorable government standards for emissions (i.e. heavier vehicles face more stringent government standards), and reduced cost associated with less material usage.
One specific problem with glass elements having reduced thickness concerns impact testing. Vehicle mirrors must pass a vehicle impact test to assure that they are durable and also to assure that they will not contribute unacceptably to flying debris during a vehicle crash. Glass elements made from existing technologies that have a thickness of less than 2.2 mm do not have sufficient strength to pass existing impact test requirements by vehicle manufacturers unless the mirror subassembly is somehow supported on its back surface or the mirror subassembly is reinforced. However, it is very difficult on a production basis to consistently provide a perfectly flat surface that non-distortingly engages and supports such thin glass elements. Also, it is difficult to provide a non-distorting reinforcement since non-uniform stresses unavoidably occur during assembly and while the mirror assembly is in service. At the same time, in conflict with the above, it is desirable to support the thin glass elements in a manner that communicates stress from the impact test against the glass elements directly back to the mirror mounting structure. If possible, this would allow the mirror mounting structure to communicate the impact stress directly to the front windshield which supports it.
Various electrochromic (EC) mirror constructions are known having features that affect impact strength and impact test results. For example, it is known to adhere foam to a rear surface of a rear glass element of an EC mirror subassembly, and to adhere a circuit board or other support structure to the foam. (See Bos U.S. Pat. No. 5,671,996.) It is known to engage a rear surface of a rear glass element of an EC mirror subassembly with rubber bumpers or metal springs or plastic springs. (See Tokai Rika prior art.) It is also known to bond front and rear glass elements together with solid-state phase electrochromic material, such that the front and rear glass elements combine to, in effect, form a single beam or plate of glass. (See Tokai Rika prior art.) However, each of the known arrangements have unwanted functional limitations or provide non-uniform support with high stress areas or "hot spots" that potentially cause unwanted deformation or distortion of thin glass elements in the range of 1.6 mm to 1.1 mm thickness or even lower. Further, no known arrangement provides a uniform non-distorting support against impact breakage across substantially an entire EC mirror subassembly, where the EC mirror subassembly uses unbonded front and rear glass elements that are 1.1 mm to 1.6 mm thick, and that are separated by a solution-phase, or liquid phase gel-type or hybrid EC material therebetween such that the thin glass elements do not directly reinforce each other.
Accordingly, a mirror assembly is desired that solves the aforementioned disadvantages and that has the aforementioned advantages.